question_answer

Refer to the common emitter amplifier circuit shown below, using a transistor with \[\beta =80\] and \[{{V}_{BE}}=0.7volt\]. The value of resistance\[{{\operatorname{R}}_{B}}\] is

A) \[330\Omega \]

B) \[330k\Omega \]

C) \[220\Omega \]

D) \[220k\Omega \]

Correct Answer: D

Solution :

\[4-{{I}_{B}}{{R}_{B}}-{{V}_{BE}}=0\]\[\Rightarrow \]\[{{I}_{B}}{{R}_{B}}=4-0.7=3.3volt\]              ...(i)

\[6-{{I}_{C}}=\left( 2.5\times {{10}^{3}} \right)-3V=0\]\[\Rightarrow \]\[{{I}_{C}}=\frac{3}{2.5\times {{10}^{3}}}=1.2\times {{10}^{-3}}\]

\[=-\frac{{{I}_{C}}}{{{I}_{B}}}=B\Rightarrow \frac{1.2\times {{10}^{-3}}}{{{I}_{B}}}=80\]\[\Rightarrow \]\[{{I}_{B}}=\frac{1.2\times {{10}^{-3}}}{800}\]\[\Rightarrow \]\[{{I}_{B}}=1.5\times {{10}^{-5}}A\]

Substituting in eq. (i)

\[1.5\times {{10}^{-5}}\times {{R}_{B}}=3.3\]

\[{{R}_{B}}=\frac{3.3}{1.5\times {{10}^{-5}}}=\frac{33}{15}\times {{10}^{5}}\]

\[=2.2\times {{10}^{-5}}\Omega \]

\[{{\operatorname{R}}_{B}}=220k\Omega \]